* multiple vmas have identical [radix_index, heap_index] value, then one of
* them is used as a tree node and others are stored in a vm_set list. The tree
* node points to the first vma (head) of the list using vm_set.head.
* multiple vmas have identical [radix_index, heap_index] value, then one of
* them is used as a tree node and others are stored in a vm_set list. The tree
* node points to the first vma (head) of the list using vm_set.head.